1. Field of the Invention
This invention is directed to an apparatus for use in the measurement of electronic devices, in general, and to an apparatus which, more particularly, uses a focused, ultraviolet (UV) light beam to provide a non-contact basis for affecting measurement and/or material modification techniques.
2. Prior Art
There are various processes which are used for fabricating electronic circuits including semiconductor devices, printed circuit boards or the like. In addition, there are many steps in these processes which require constant testing and/or monitoring. Moreover, there are many instances where testing is required of the ultimate device, which testing cannot be made during the processing steps. This is especially the situation in regard to semiconductor devices. Likewise, there are times when the product, notably relative to a semiconductor device, is sufficiently complex and/or dense so that it is impossible to make internal probes or tests which are not destructive of the device.
In the past it has frequently been the case that a scanning electron microscope (SEM) has been used in conjunction with these testing procedures. In particular, the SEM is used for examination of process steps, somewhat analagous to a high powered microscope. The SEM is used mostly for examining the devices for thinning of metal over oxide steps (in semiconductor components). However, the SEM is used virtually not at all for actual electrical testing of "in-process" or even fully processed semiconductor devices. That is, the SEM is used primarily for failure analysis or examination of defects. The SEM is used to measure the variation in yield of electrons from the device in order to control image quality. This can be a function of the quantity of secondary electrons which are emitted from the surface of the device under examination. However, the cost of an SEM is quite high and, therefore, sometimes prohibitive. Moreover, the SEM is frequently not capable of making tests because the SEM has two main problems when trying to measure voltage or when performing examination on integrated circuit devices. The first is the fact that the SEM uses a beam of focused electrons that strike the sample. These electrons build up on the insulation parts of the integrated chip. This charging causes malfunctions of sensitive circuits so that a valid test cannot be made. Another effect of charging by the SEM electron beam is distortion of both the image (which is made from secondary electrons that leave the sample) and the ability to make voltage measurements.
Further, even without the undesirable charging effects, the SEM is incapable of making accurate quantative voltage measurements because the yield of secondary electrons produces a broad spectrum of energy. Attempts to measure the voltage on a conductor by detecting and "nulling" this broad spectrum of energies actually produces an average-not an actual measurement.
Other types of non-contact measurements have been developed using Auger spectroscopy techniques. In particular, this technique is established when a sample is biased to a normal operating mode and a shift in the energy of the Auger electrons is recorded. The shift in the Auger electron energy in eV is, of course, directly proportional to the potential (in volts) at the site being analyzed. This shift is known to be due to either chemical bonding or a change in the bias between the sample and the electron detector. By eliminating the chemical bonding as the cause of energy shift, it is determined that the bias on the detector or the sample has caused the shift.
As suggested above, the SEM technique includes a known phenomenon wherein the electric potential of the electron probe illuminated position is different from that of the surrounding area. In this event, the number of secondary electrons is different from that of the surrounding area and produces the voltage contrast. That is the voltage contrast in a SEM is produced by the effect of the change in the yield of secondary electrons from the site struck by the primary electron beam. Moreover, regions of the circuit that are at more negative voltages yield more electrons and appear brighter in the image. Circuit regions at more positive voltages yield fewer electrons and appear darker in the SEM image. Unfortunately, this ability to measure the voltage is degraded by the charging and by the broad range of electron energy mentioned above. Use of a stroboscopic SEM improves the operation of this apparatus somewhat but it is very expensive and difficult to accomplish.
Another technique is eletron beam probing, especially in IC (integrated circuit) testing. This technique requires the use of an SEM which has been modified to include a blanking circuit, a timing unit, an IC drive unit, a signal processing and display unit, and a secondary-electron spectrometer if the measurement is to be truly quantative. The spectrometer is required because the electron beam produces a broad, continous range of energies. In contrast, the UV source produces a predictable maximum energy of secondary electrons. In essence, electron beam probing is similar to the voltage contrast technique noted above except that the electron spectrometer is used. This technique has the same problems as the standard SEM, viz. charging of the sample and the broad range of electron energies produced by the primary electron beam.
These are several techniques which are known in the prior art. However, each of these techniques and processes has various shortcomings as noted above. Consequently, an improved measuring and detecting device and technique is desired.